Hi, personally I wouldn’t go for a Core as there are too many things the Unitiserve does, that the Core can’t, for it to be worthwhile in my view.
The sound quality I get from my Synology NAS running a server, using the backup music folders from my Unitiserve, sounds identical to the US itself, as did the Core and an Innuos server I had on demo a while back. From a functional point of view I did like the Innuos but I have yet to hear anything that sounds significantly different to, or better than the basic Synology NAS which today would probably cost under £300.
Are your copies on your computer WAV or FLAC? You may find that as WAVs they have metadata that can’t be read by non-Naim devices. There’s some software that may be able to do this but a regular file converter can’t.